A technology for integrating metals and synthetic resins is needed in a wide range of industrial fields including the production of parts for automobiles, domestic electric appliances, industrial machinery, and the like, and a large number of adhesives have been developed therefor. Among them, excellent adhesives have been developed. For example, adhesives demonstrating adhesive functions at normal temperature or under heating are used for integrally joining metals and synthetic resins, and such a method is presently a generally employed joining technology.
However, more rational joining methods that use no adhesives have also been studied. An example of such methods is a method for integrating high-strength engineering plastics, without using an adhesive, with light metals such as magnetism, aluminum, and alloys thereof or iron alloys such as stainless steel. For example, the inventors have suggested a method for injecting a molten resin onto a metal part that has been inserted in advance into an injection molding mold, molding the resin part and simultaneously fixing (joining) the molded part and the metal part (this method will be referred to hereinbelow as “injection joining”).
This invention suggests a manufacturing technology by which a polybutylene terephthalate resin (referred to hereinbelow as “PBT”) or a polyphenylenes sulfide resin (referred to hereinbelow as “PPS”) is injection joined to an aluminum alloy (for example, see Patent Document 1). A joining technology has also been suggested by which a large hole is provided in an anodization film on an aluminum material and bonding is performed by filling the hole with a synthetic resin body (for example, see Patent Document 2).
The principle of the injection joining suggested in the Patent Document 1 will be explained below. An aluminum alloy is immersed into a dilute aqueous solution of a water-soluble amine compound, and the aluminum alloy is finely etched by a weak basicity of the aqueous solution. Further, it has been found that the adsorption of the amine compound molecules on the aluminum alloy surface simultaneously occurs during the immersion. The aluminum alloy subjected to such treatment is placed as an insert into an injection molding mold and a molten thermoplastic resin is injected under a high pressure.
In this case, heat is generated by the interaction of the thermoplastic resin and amine compound molecules adsorbed on the aluminum alloy surface. Almost simultaneously with this heat generation, the thermoplastic resin comes into contact with the aluminum alloy that has been held at a low temperature of the mold and is rapidly cooled. As a result, solidification of the resin that has to crystallize and solidify is delayed and it penetrates into ultrafine concavities present on the aluminum alloy surface. As a consequence, the aluminum alloy and thermoplastic resin are strongly joined (fixed), so that the resin does not peel from the aluminum alloy surface. Thus, where the exothermal reaction occurs, strong injection joining becomes possible. It has actually been confirmed that PBT or PPS that can participate in exothermal reactions with amine compounds can be injection joined to aluminum alloys.    Patent Document 1: Japanese Patent Application laid open No. 2004-216425    Patent Document 2: International Patent Application WO2004/055248 A1